Recovery of benzene and benzene derivatives from gasoline fractions and refinery streams

ABSTRACT

A process for the separation of the aromatic compounds benzene, toluene and xylene from an aromatics-containing reformate gasoline and pyrolysis gasoline or a coke-oven light oil or an aromatics-containing refinery stream, in which the aromatics are separated by an extractive distillation uses a novel solvent combination made up of the compounds n,n′-diformyl piperazine or 2,2-bis-(cyanoethyl)ether in a combination with n-formyl morpholine as a second solvent for extractive distillation so that the solvent combination obtained shows a higher selectivity with regard to the aromatics to be extracted so that a lower solvent load is required. The aromatics-containing feed mixture is first submitted to a pre-distillation so that the obtained fraction has a narrow boiling point range. This fraction is then submitted to an extractive distillation in a first column, in which an aromatics-lean head product of predominantly paraffinic hydrocarbons is obtained as well as an aromatics-enriched bottom product. The bottom product is passed to a second column in which an aromatics-rich raffinate is obtained by reducing the pressure or increasing the temperature so that the extracting solvent combination obtained as bottom product can be recycled into the process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.12/733,204, filed May 27, 2010, which was a US National Phaseapplication of International Application PCT/EP2008/006415, which has anInternational Filing Date of Aug. 5, 2008. U.S. Ser. No. 12/733,204 waspending as of the filing date of this application, and PCT/EP2008/006415was pending as of the filing date of U.S. Ser. No. 12/733,204, and eachof these applications is expressly incorporated by reference as if setforth in their entirety herein.

BACKGROUND OF THE INVENTION

The recovery of aromatics from gasoline fractions and refinery streamsis an important process step in petrochemistry and coke over andrefinery technology. Especially benzene and simple derivatives ofbenzene are important raw materials in the production of dyestuffs,plastics, solvents and varnishes. As these compounds inaromatics-containing fractions frequently occur in mixtures withnon-aromatic compounds, process steps for their isolation are of greatimportance. Examples of fractions containing aromatics are reformategasoline and pyrolysis gasoline but also distillation fractions frommineral oils or coke-oven light oil.

It is not possible to simply separate aromatics from gasoline fractionscontaining aromatics by way of distillation since the gasoline or itsfractions consist of a large variety of substances of very similarboiling ranges. The separation is therefore to be carried out byprocesses which take advantage of other physical effects. The technicalimplementation can be achieved by several processes based on differentphysical separation processes. To be mentioned here in the first placeare the azeotropic distillation, the liquid/liquid extraction and theextractive distillation.

In the azeotropic distillation, a solvent is added to the mixture to beseparated, this solvent forming a mixture of constant boiling point withthe aliphatic or the aromatic component. This azeotrope is separatedfrom the starting mixture by distillation and after distillationfractionated into azeotrope former and aromatic fraction. In theliquid/liquid extraction, the mixture to be separated is provided with asolvent generating a two-phase mixture and involving a higher solubilityof one component and thus extracting it from the solvent mixture. Thearomatic component may be separated from the solvent, for example, byway of distillation after the extraction.

The extractive distillation takes advantage of the phenomenon that thereis a change in the fugacities in a mixture of appropriate components.The fugacity is here to be understood as the corrected partial vapourpressure in the mixture. Reason for the change in fugacity is the factthat there are different repellent interactions between the individualtypes of molecules. A mixture component that has stronger repulsiveforces than the other components will change into the vapour phase moreeasily than a component of lower repulsive forces.

In an extractive distillation a solvent is added which is known to becapable of selectively increasing the fugacity of one or severalcomponents. In the case of hydrocarbon mixtures containing aromatics,the aliphatic components of the mixture frequently have strongerrepulsive forces vis-á-vis the solvent so that their fugacity is henceconsiderably increased. In contrast to this, the fugacity of thearomatic component changes comparatively less. For this reason, adistillation with the solvent will effect that the aliphatic componentsare preferably obtained in the raffinate, the low-boiling head productof a distillation, whereas the aromatic components are obtained in theextract, the higher-boiling bottom product of a distillation. This makesit necessary to use a solvent achieving the desired effect by changingthe fugacities of the individual components in the desired way.

An extractive distillation frequently has advantages over an azeotropicdistillation or a liquid/liquid extraction. The mass transfer in anextractive distillation frequently is considerably higher than in anazeotropic distillation as, in the case of the former, the appliedtemperature are distinctly higher. An extractive distillation requiresconsiderably less equipment than a liquid/liquid extraction, as only twodistillation columns are usually required instead of one extractioncolumn with downstream distillation unit. As considerably less solventis required for the extractive distillation as compared to aliquid/liquid extraction, the costs of installation and operation arenotably lower.

The central problem to be solved when performing an extractivedistillation is the selection of a suitable solvent. From the largenumber of possible solvents the one is to be determined that allows theintended separation with a minimum amount of circulated solvent.Decisive criteria for this are the capacity and the selectivity of asolvent. The capacity indicates how the aromatic component in liquidstate is distributed among the individual phases according to Nernst'sdistribution law. The higher the capacity, the better the solubility ofthe aromatic component in the solvent and the lower the solvent demand.The lower the capacity, the higher is the probability that a two-phasemixture is formed with the aromatic components and the solvent in liquidstate. Hence the capacity mainly determines how much solvent isrequired.

The selectivity indicates the improvement of the extraction of thedesired transition component in comparison to the other componentscontained in the raffinate. The higher the selectivity of an extractingagent, the stronger is the repulsion of the aliphatic component and thusthe corresponding change in fugacity. The selectivity essentiallydetermines the separating efficiency and thus the number of theoreticaltrays required for the extractive distillation. The lower theselectivity, the more equipment is required.

Various solvents suited for an extractive distillation of aromatics areknown. Frequently used solvents are diethylene glycol,dimethylsulfoxide, sulfolane, n=methyl pyrrolidone, dimethylacetamideand n-formyl morpholine. A hydrocarbon stream is used as a feed mixture,which contains aromatic and aliphatic components and is distilled in apre-distillation unit to give a hydrocarbon with a relatively narrowboiling point range. Depending on the separating efficiency of theextractive distillation, the feed stream is a C₆-stream, a(C₆-C₇)-stream or a (C₆-C₈)-stream.

The actual equipment for the extractive distillation usually consists oftwo distillation columns. The first column serves to perform the actualextractive distillation. At the head of the column a raffinate stream isobtained which predominantly consists of non-aromatic hydrocarbons and,depending on the configuration, a certain amount of solvent. As therepulsive effect of the solvent is stronger for the non-aromatichydrocarbons, these compounds change to the vapour phase more easily. Atthe lower part of the column a mixture is obtained which predominantlyconsists of aromatic compounds and the extracting solvent. This mixtureis then passed into a stripping column where the aromatics-containingmixture is separated from the solvent by way of distillation. Thesolvent is recycled back to the first column.

The distillative separation of the extract gives a hydrocarbon mixturerich in aromatics as a fraction at the head of the stripping column anda solvent fraction as bottom product which is lean in aromatics. Bothfractions may be passed to a downstream purification. Once purified, thearomatics may, for instance, be further processed by distillation toobtain the individual aromatic compounds by alkylation degree andboiling point. Thus the benzene derivatives benzene, toluene and xyleneare obtained. To separate the xylene isomers, further process steps mayfollow. As a purification step for the aromatics fraction, a scrubbingprocess with water may be advisable.

DE 1568940 C3 describes a process for the extractive distillation ofaromatics using n-formyl morpholine as a solvent. The process can beused for the isolation of aromatics from a starting fraction containingaromatics as well as for the removal of aromatics from hydrocarbonstreams. This process is run in a facility including a column forextractive distillation, a solvent stripper, a stripping column and asolvent regeneration column. Depending on purity and requirements, thearomatics contained can be obtained either directly or be submitted tofurther treatment. Owing to the relatively low solvent capacity thisprocess requires a high amount of solvent and involves highconstructional cost.

EP 679708 A1 describes an extraction process which requires only oneextraction column owing to a specific equipment arrangement. Theextraction is carried out in a column from which a head product rich inaliphatic compounds is obtained and a side product which is rich inaromatics from the middle column section. The solvent is recycled fromthe bottom via heat-exchanging devices to the upper column section. Bothhydrocarbon streams are freed from excessive solvent and water incyclone separators and downstream phase separators. As solvents,preferably polyalkylene glycols are used but also sulfolanes orpyrrolidones. To improve the separating efficiency, 0.1 to 20 masspercent of water are added to the solvent mixture. A disadvantage ofthis process is that the separating efficiency can only be improved if acertain amount of water is added. This makes it necessary to installadditional devices for drying the products obtained.

EP 1280869 B1 describes a process for the extractive distillation of ahydrocarbon mixture containing aromatics using a solvent mixture ofsulfolane and 3-methyl sulfolane. The solvent mixture can be used in anyratio desired and thus adapted to the aromatics content and thecomposition of the aromatics portion to an optimum degree. This processis run in a facility consisting of a column for extractive distillationand a column for distillation of the aromatic fraction. By sucharrangement the process can be run with a relatively low demand forequipment. The disadvantage of this process is a large amount ofcirculated solvent and a relatively large column for extractivedistillation, as a large portion of extracting solvent combination ascompared to the hydrocarbon must be used.

SUMMARY OF THE INVENTION

The aim of the invention is to find a solvent and a suitable process forthe extractive distillation of benzene derivatives from gasolinefractions, which is characterized by improved properties as regards theaspects of selectivity and capacity. The capacity of the solvent shouldbe adequately high so that a relatively little amount of circulatedsolvent is required. The solvent cost should be low and the demand forequipment low. The achieved selectivity of the solvent should be high soto ensure adequate shifting of the boiling point of the aliphaticcomponent and easy isolation of the paraffinic and aromatic hydrocarbonsfrom the gasoline fraction.

The present invention achieves the aim by using a new solvent mixturewhich meets the specified requirements. It was found that the twosolvent additives n,n′-bis-(formyl)piperazine or 2,2′-bis-(cyanoethyl)either in combination with n-formyl morpholine are especiallywell-suited for the extractive distillation of aromatics. The capacityof this solvent combination is so high that the aromatics can beseparated from the gasoline fraction with an only small amount ofextracting agent. The selectivity in relation to aromatics is so highthat, after addition of the extracting solvent, the boiling points ofthe aromatic compounds differ sufficiently enough for a distillativeseparation. By the high selectivity of the solvent combination accordingto the invention, the non-aromatic hydrocarbons are almost completelyseparated from the gasoline fraction so that, after the removal of theextracting solvent, it will be possible to carry out a simpledistillation separation of the aromatics benzene, toluene and of thexylenes.

By the variable selectivity-increasing effect of the additional solventcomponents according to the invention, it is possible to reduce theamount of circulated solvent mixture to an extent that the capacitylimit or, in other words, the phase separation of solvent and aromaticsmixture is almost reached. The solvent combination can be optimized suchthat it will bring a maximum saving of solvent load for every gasolinefraction. A lower amount of circulated solvent involves lower investmentcost and utility cost with plant capacity and product purity remainingthe same.

DETAILED DESCRIPTION OF THE INVENTION

Claimed is a process for the recovery of a pure aromatics fractioncontaining benzene, toluene or xylene or mixtures of these aromaticsfrom refinery streams or gasoline fractions containing such aromatics byway of extractive distillation, in which

-   -   the feed product is subjected to a pre-distillation in a first        process step prior to the extractive distillation, where the        components which boil at higher temperatures than the mentioned        aromatics are separated as a bottom product, and    -   the thus obtained starting mixture containing aromatics is mixed        in a second process step with an extracting solvent or solvent        mixture which is capable of selectively increasing the fugacity        of the non-aromatic components of the starting mixture and thus        the separating efficiency, and submitted to an extractive        distillation, and    -   the extracting solvent from the obtained extract is distilled        off in a third process step by increasing the temperature or        increasing the temperature and reducing the pressure,        characterized in that    -   the extractive distillation of the second process step is        carried out using a solvent combination comprising n,n′-diformyl        piperazine with another solvent or 2,2′-bis-(cyanoethyl)ether        with another solvent as vapour-pressure changing solvent        combination.

The solvents according to the invention may be used either in a mixtureor as individually as solvent components. In a preferred embodiment theyare used in a mixture with a solvent which is not in accordance with theinvention. A preferred solvent combination is n-formyl morpholine in amixture with one of the two solvent components according to the presentinvention.

By the novel solvent combination it is possible to save a notable amountof circulated solvent. If a solvent combination of the solvents n,n′-diformyl piperazine and n-formyl morpholine in a mass ratio of 1:1 isadmixed, 10 to 30 mass percent of circulated solvent can be saved ascompared to the pure solvent n-formyl morpholine. In a preferredembodiment of the invention, the saving in the amount of circulatedsolvent ranges between 15 and 25 mass percent.

if a solvent combination of the solvents 2,2′-bis-(cyanoethyl)ether andn-formyl morpholine in a mass ratio of 1:1 is admixed, 5 to 15 masspercent of circulated solvent can be saved as compared to the puresolvent n-formyl morpholine. In a preferred embodiment of the invention,the saving as compared to the solvent n-formyl morpholine ranges between7 and 11 mass percent.

N,n′-diformyl piperazine or hexahydro-1,4-diazine-1,4-dimethanal(HCO[cyclo-N(CH₂CH₂)₂N]CHO) is a chemical substance which is easy toprocure and frequently used for the recovery of fine chemicals.3,3′-oxydipropionitril or 2,2′-bis-(cyanoethyl) ether(NC(C₂H₄)O(C₂H₄)CN) is a chemical substance which is easy to procure andfrequently used for the recovery of fine chemicals.3,3′-oxydipropionitril or 2,2′-bis-(cyanoethyl)ether (NC(C₂H₄)O(C₂H₄)CN)is a chemical substance which is easy to procure and frequently used asa solvent in chromatography owing to its specific polar characteristics.This chemical can be produced easily so that larger amounts of thecompounds are available at reasonable price. The melting point of thecompound n,n′-diformyl piperazine which is solid at room temperature isat 125 to 129° C., melting and boiling point of the compound2,2′-bis-(cyanoethyl)ether which is liquid at room temperature is at−26° C. and 130° C. (0.26 kPa).

To run the process according to the invention the two solvents accordingto the invention are preferably used in combination with a secondsolvent in order to keep the foiling point of the extracting solventcombination within a range suitable for the separation of the solvent.If the boiling point is too high, it is possible that the extractingsolvent decomposes when the solvent is separated from the aromatics. Theaddition of a solvent not in accordance with the invention lowers theboiling point of the solvent combination according to the invention sothat a decomposition of solvent components can be avoided without beingforced to keep the pressure at a low level.

In another embodiment of the invention, substituted heterocycliccompounds containing nitrogen and oxygen are added as a second solvent.Especially suitable as a second solvent is n-formyl morpholine. Thesecond solvent can be added to the solvent according to the invention ingreatly varying weight percentages to achieve the effect according tothe invention. Preferred for running the process according to theinvention is a ratio of the two solvents of 1:1.

In a further embodiment of the invention, the solvent componentsaccording to the invention are used in the form of derivatives. Thus itis possible, for example, to introduce carbonaceous substituents intothe solvent component according to the invention without any essentialchanges of the characteristics responsible for the extraction. To keepthe solubility of the extracting solvent in a range suitable for theprocess according to the invention, the number of the C atoms of allsubstituents is not larger than 7.

Claimed in addition to the described process for extractive distillationis a substance mixture which consists in the compounds n,n′ diformylpiperazine and n-formyl morpholine. Also claimed is a substance mixturewhich consists in the compounds n,n′ diformyl piperazine and2,2′-bis-(cyanoethyl)ether. The mixtures have not yet been provided ordescribed as a substance in this form. Claimed as well is the use of thesubstance mixtures according to the invention for the extractivedistillation.

The process according to the invention is run by means of an apparatustypical of the extractive distillation of aromatics. An exemplaryapparatus is described in EP 434959 A2. The aromatics-containing feedmixture obtained from the pre-distillation of the gasoline is pre-heatedand fed into the bottom part of a first distillation column provided forextraction. The latter has already been loaded with the extractingsolvent combination according to the invention. In the extractivedistillation, a raffinate stream is obtained at the head of the column,which contains a considerably depleted amount of aromatic hydrocarbonsand essentially contains paraffinic hydrocarbons or perhaps a certainamount of naphthenic hydrocarbons. This raffinate stream contains a verylittle amount of extracting solvent only and, as soon as, for example, ascrubbing unit. As a bottom product of the extraction column, an extractstream strongly enriched with aromatic hydrocarbons in a mixture withthe extracting solvent. In an embodiment of the invention, theextractive distillation is carried out at reduced pressure.

The extract stream is fed into the bottom part of the seconddistillation column which is also referred to as the stripping column.The latter is provided for the distillative separation of the solventfrom the aromatics to be obtained. As a bottom product of this column,the solvent is recovered and recycled via a line into the upper part ofthe first column which is provided for the extractive distillation. Inthis way it is possible for establish an essentially closed loop for theextracting solvent combination. To heat the two columns, they mayoptionally be equipped with a reboiler circuit.

As a head product of the second column provided for solvent separation,a hydrocarbon stream essentially free of solvent is obtained, whichprimarily contains the desired aromatic hydrocarbons. As soon asobtained, this aromatic stream can be transferred to a downstreamtreatment unit.

In an embodiment of the invention, the aromatic stream obtained from thesolvent separation is transferred to an optional scrubbing process withwater to remove any remainders of the solvent combination obtained. Thescrubbing process may be followed by further processing steps in phaseseparators. Upon receipt, the solvent-free extract produced ispreferably submitted to a distillative separation, in which theindividual benzene derivatives are obtained in an amount whichcorresponds to the portion of the individual aromatics in the startingmixture. A specific advantage of the process according to the inventionis that it is possible in this distillation to obtain the xylenes, whichare harder to obtain, as a pure xylene fraction.

In a further embodiment of the invention the separation of theextracting solvent from the aromatics is performed at reduced pressure.In this way it is possible to limit the thermal load of the extractingsolvent combination and to reduce the equipment required for cooling theextract stream. To allow that the solvent be separated in the seconddistillation column at reduced pressure, the gas pressure of theraffinate stream obtained from the first column provided for theextraction is decreased by a suitable device.

The effect according to the invention could be supported by theoreticalVLE (vapour liquid equilibrium) data-based calculations. The processaccording to the invention was simulated in a process which wascalculated by entering the main plant parameters into the Aspen Pluscomputer program of Aspen Tech make. Input data were temperature,pressure, boiling point, interaction parameters and solubility of allspecified compounds. Used in the simulated process was a distillationunit which was typical of an extraction process for single aromatichydrocarbons. To demonstrate the effect according to the invention, thepure solvent n-formyl morpholine was compared with the solventcombinations according to the invention, i.e. n,n′-diformyl piperazinewith n-formyl morpholine and 2,2′-bis-(cyanoethyl)ether with n-formylmorpholine. Both solvent combinations were specified with a mass rationof 1:1 in the simulation calculation. The solvent amounts which wereobtained as a result at important process points were converted intopercent by weight for the calculated saving in the amount of circulatedsolvent. The fault tolerance of this computer program amounted to ±10%for the percent figures of solvent saving.

To achieve an advantageous energetic balance of the process, the hotbottom product obtained from the solvent separation, which essentiallyconsists of extracting solvent, can be recycled to the process viaheat-exchanging devices. The hot bottom product from the strippingcolumn may be used for heating the feed mixture, for heating the firstcolumn provided for extractive distillation or for heating the secondcolumn provided for solvent separation.

In the event of continuous operation of the plant for the recovery ofaromatics, a slight loss in extracting solvent may occur despite all themeasures taken. Solvent may get into the downstream treatment processesvia the raffinate streams from the low-boiling fractions. To compensatesuch loss, pre-heated fresh solvent may be fed to the process bysuitable devices at the head of the first column.

The fed amount is controlled such that the weight ratio of used solventto hydrocarbon mixture used ranges between 1:1 and 5:1. In a preferredembodiment of the invention the weight ratio of the used solventcombination and the used hydrocarbon mixture ranges between 2:1 and 3:1,provided the solvent components according to the invention are used in amixture with n-formyl morpholine.

In an embodiment of the invention, the distillation temperature in thefirst column provided for extraction is adjusted such that the raffinatedischarged at the head of the column is of a temperature of at least 50°C. at atmospheric pressure and the bottom product obtained at the lowerend of the column is of a maximum temperature of 200° C. The boilingtemperatures inside the column may change if the distillation pressureor the composition of the hydrocarbon mixture changes.

In another embodiment of the invention, the temperature in the secondcolumn provided for solvent separation is adjusted such that theraffinate discharged at the head of the column is of a temperature isessentially determined by the decomposition temperature of the solvent.The boiling temperatures inside the column may change if thedistillation pressure or the composition of the hydrocarbon mixturechanges.

In a further embodiment the solvent is provided with a low water portionto increase the selectivity of the extractive distillation. The watercontent may range between 0.1 and 20 mass percent, preferably, however,between 0.5 and 10 mass percent. This depends on the process andequipment conditions selected for the respective device.

The process according to the invention can be used for isolatingaromatics from an aromatics-containing starting fraction as well as forpurifying hydrocarbon streams from aromatics. The removal of aromaticcompounds from a mainly paraffinic mixture may be of interest to thefoodstuff industry, for example. As starting gasoline fractions,hydrocarbon streams from mineral oil processing and from refineries maybe used as well as products obtained from coke oven or hydrocarbonproducing plants as, for instance, coke-oven light oil.

Running the process according to the invention does not require anyessential changes in equipment as compared to conventional extractivedistillation processes. The capacity of the solvent used risessignificantly so that altogether less solvent is required forcirculation in the overall loop. The solvent components used accordingto the invention reduce the amount of circulated solvent and thus reducethe cost as well. The lower amount of circulated solvent brings aboutlower investment cost while plant capacity and product purity remain thesame. The selectivity of the solvent mixture also increasessignificantly so that it is possible to adapt the solvent optimally tothe aromatics content and the aromatics distribution in the feedmixture.

The configuration of the process according to the invention for therecovery of benzene derivatives from gasoline fractions and refinerystreams is illustrated in more detail by means of an example and adrawing, the process according to the invention not being restricted tothis embodiment.

Example: The attached table (Table 1) outlines the saving of solvent inthe process according to the invention using the solvent combinationaccording to the invention as compared to the conventional n-formylmorpholine.

TABLE 1 Saving of solvent as compared Solvent combination (mass ratio)to n-formyl morpholine N,n′-diformyl piperazine + 10 to 30 mass percentn-formyl morpholine (1:1) 2,2′-bis-(cyanoethyl)-ether +  5 to 15 masspercent n-formyl morpholine (1:1

The attached drawing (FIG. 1) shows an exemplary embodiment of theprocess for extracting aromatics from a gasoline fraction.

Via a feed line 1, an aromatics-containing feed mixture obtained from apre-distillation unit is fed to the middle section of a column 2 whichis provided for the extractive distillation of the aromatics-containingmixture. The column has already been loaded with the solvent accordingto the invention in combination with another solvent. The distillationproduces a raffinate which is an aromatics-lean hydrocarbon stream 3mainly containing paraffinic hydrocarbons and being passed to furtherprocessing. The bottom product obtained is an aromatics-enrichedhydrocarbon stream 4 which is passed via a line with pressure-reducingdevice 5 to a second column 6 for solvent separation. In this column thesolvent is separated by increasing the temperature and/or reducing thepressure. As a head product, an aromatics-rich extract 7 is obtainedwhich is largely free of solvent. As a bottom product, a solvent streamis obtained which mainly contains the aromatics-lean solvent. This isreturned via a line 8 into the upper part of column 2 provided forextractive distillation. Via indirect heat exchange by means ofheat-exchanging devices 9, 10 and 11, the solvent can be used to heatstarting mixture 1, extraction column 2 or stripping column 6. By addingsolvent or solvent components via a separate feed nozzle 12 it ispossible to compensate the loss in solvent during continuous operation.

LIST OF REFERENCE NUMBERS AND DESIGNATIONS

-   1 Starting mixture feed line-   2 Extractive distillation column-   3 Raffinate, product stream of aromatics-lean hydrocarbon mixture-   4 Line for aromatics-rich solvent mixture-   5 Pressure-reducing device-   6 Stripping column for the separation of solvent-   7 Product stream of aromatics-rich hydrocarbon mixture-   8 Solvent recycle line-   9 Heat exchanger-   10 Heat exchanger-   11 Heat exchanger-   12 Solvent feed nozzle

What is claimed is:
 1. A method of utilizing a mixture of n-formylmorpholine and n,n′-diformyl piperazine in any composition desired,wherein it is employed in an extractive distillation of anaromatics-rich hydrocarbon mixture.
 2. A method of utilizing a mixtureof n-formyl morpholine and 2,2′-bis-(cyanoethyl)ether in any compositiondesired, wherein it is employed in an extractive distillation of anaromatics-rich hydrocarbon mixture.
 3. In an extractive distillationprocess, exposing an aromatics-rich hydrocarbon mixture to a solventmixture comprising n-formyl morpholine and n,n′-diformyl piperazine,each solvent being present in the mixture in any desired proportion tomaximize the selectivity of the solvent mixture.
 4. In an extractivedistillation process, exposing an aromatics-rich hydrocarbon mixture toa solvent mixture comprising n-formyl morpholine and2,2′-bis-(cyanoethyl), each solvent being present in the mixture in anydesired proportion to maximize the selectivity of the solvent mixture.